FIELD OF THE INVENTION
The invention lies in the field of exhaust gas purification. In particular, the invention pertains to a method of reducing the NO.sub.x content in the exhaust gas of a diesel internal combustion engine. The applicable system includes a catalytic converter for the catalytic after-treatment of the exhaust gas according to the SCR principle. A reducing agent is supplied into the exhaust gas upstream of the catalytic converter. A control unit determines the control parameter of the diesel engine, in particular the start of injection of the fuel, the idling speed, and the respective catalytic-converter efficiency and calculates the desired value for the reducing agent feed in dependence on the catalytic-converter efficiency.
The selective-catalytic-reduction method, or SCR method, is suitably used to reduce the NO.sub.x content in the exhaust gas of a diesel internal combustion engine. In the process, a reducing agent is injected into the exhaust gas at a point upstream of a catalytic converter. Any NO.sub.x contained in the exhaust gas is thus reduced to N.sub.2. Ammonia may be used as the reducing agent. For reasons of ease-of-handling, an aqueous solution of urea is normally used. See, for example, U.S. Pat. No. 5,665,318 (German patent application DE 44 36 415 A).
In the urea SCR system, the desired quantity of the reducing agent metering is continuously calculated by the control unit of the diesel internal combustion engine. To this end, the control unit requires the instantaneous catalytic-converter efficiency. The latter is determined from operating parameters and the catalytic-converter temperature. The catalytic-converter temperature may in turn be determined, for example, by a model computation from the exhaust gas temperature and the exhaust gas mass flow.
The effectiveness and efficiency of the exhaust gas treatment depends on the catalytic-converter temperature. FIG. 3 shows a typical characteristic of the catalytic-converter efficiency as a function of the catalytic-converter temperature. The catalytic-converter efficiency at which the NO.sub.x reduction is effected reaches a maximum within the temperature range around 250.degree. C. and drops steeply toward lower temperatures. The catalytic-converter efficiency approaches zero at temperatures below 120.degree. C. If aqueous urea solution is used as the reducing agent, the metering may only be effected if the exhaust gas temperature is high enough for the decomposition of urea for forming the ammonia required for the catalytic conversion. The decomposition of urea is catalytically assisted and starts at temperatures above 130.degree. C. Lower temperatures mean, therefore, that, firstly, no reducing agent can be metered and, secondly, the catalytic converter is ineffective.
The catalytic exhaust gas treatment in a diesel internal combustion engine, specifically for the NO.sub.x reduction, is ineffective directly after a cold start of the engine. The catalytic reaction does not start until the exhaust gas temperature increases. During coasting operation, the exhaust gas cools down relatively quickly, so that, for instance during the acceleration of a vehicle after downhill sections, the catalytic NO.sub.x reduction is virtually ineffective, which results in an undesirably high NO.sub.x emission.